Plasma CVD method

ABSTRACT

In a process of forming a silicon oxide film  116  that constitutes an interlayer insulating film with TEOS as a raw material through the plasma CVD method, the RF output is oscillated at 50 W, and the RF output is gradually increased from 50 W to 250 W (an output value at the time of forming a film) after discharging (after the generation of O 2 -plasma). A TEOS gas is supplied to start the film formation simultaneously when the RF output becomes  250  W, or while the timing is shifted. As a result, because the RF power supply is oscillated at a low output when starting discharging, a voltage between the RF electrodes can be prevented from changing transitionally and largely.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma CVD method used formanufacturing a semiconductor integrated circuit such as a thin-filmtransistor.

2. Description of the Related Art

At the present time, as the semiconductor integrated circuit is madehigh in integration as well as density, there is advanced that thestructure of a semiconductor element is made finer. Under thiscondition, there is such a demand that an interlayer insulating film notonly has an insulating characteristic, but also can be filled closelybetween wires which are complicated and have high aspects. Up to now,since a silicon oxide film which is made of TEOS as raw material andformed through the CVD method is excellent in coating shape, it has beenwidely used as an interlayer insulating film. In particular, the plasmaCVD method is applied since it enables the silicon oxide film to bemanufactured at a low temperature of 400° C. or lower and also enables alarge area to be processed, in a process of manufacturing a TFT which isto be formed on a glass substrate.

However, as the semiconductor integrated circuit is made high inintegration as well as density, the influence of charge-up of electronscaused during a plasma process becomes remarkable. For example, it ispresumed that the failure of U-shaped display and the defect of pointsare caused by electric damages during the process. In the active matrixtype liquid-crystal display unit, the failure of one TFT means thefailure of an entire panel, thereby leading to the deterioration of ayield.

SUMMARY OF THE INVENTION

In order to eliminate the above problem, an object of the presentinvention is to provide a plasma CVD method that is capable ofsuppressing the deterioration of a device which is caused by chargeparticles.

The process by which the present invention has been achieved will bedescribed.

In the plasma CVD device, in a state before plasma is developed, avoltage applied from an RF electrode acts as an electric field for asubstrate. It is presumed from the viewpoints of an interval between anelectrode and a wire, the thickness of a substrate, etc., that in thisstate, the intensity of an electric field is not so much as a deviceformed on the substrate is destroyed.

On the other hand, due to charge particles (electrons and positive ions)are produced in the process of generating plasma, a space between the RFelectrodes becomes conductive. A substrate surface starts to benegatively charged with respect to plasma due to a difference inmobility between electrons and positive ions (the generation of ionsheath). Thereafter, the amount of generation of charge particles isbalanced with the amount of disappearance of charge particles, that is,the amount of charge-up of charge particles is saturated, resulting in astationary plasma-state.

Hence, it is presumed that because the ion sheath produced on thesubstrate surface is not considered to become an excessively value, theintensity of electric field and ion irradiation energy are not so muchas they destroy the device.

However, there is the possibility of allowing current to flow in aportion where little current flows in the stationary state until theamount of charge-up is saturated in a moment when plasma is generated,and if a large current flows in that portion transitionally, the deviceis then destroyed instantly.

Up to now, in order to form a silicon oxide film which is made of TEOSas raw material through plasma CVD, two processes consisting of apre-process for generating plasma and a process of supplying TEOS toform the film are continuously conducted (in a state where O₂ plasma isbeing generated).

To elucidate the transitional phenomenon in the plasma CVD, the presentinventors, et al. have observed the waveform of voltage applied betweenthe RF electrodes with the connection of an oscilloscope to an RF powersupply. FIG. 5 shows the waveform of voltage applied between the RFelectrodes in a conventional film forming process, and the unit of avertical axis is 200 V/div whereas the unit of a horizontal axis is 500msec/div. In both of the plasma generating process and the film formingprocess, an RF output is 250 W. FIGS. 6A to 6C show the waveforms ofvoltage applied between the RF electrodes in an O₂ plasma generatingprocess where the RF output is 250 W. FIG. 6A shows the waveform at thetime of starting oscillation, and FIGS. 6B and 6C show the waveforms atthe time of starting discharge, respectively. The instant that thedischarge starts can be recognized as the shift of waveforms. It shouldbe noted that the unit of the vertical axis of FIGS. 6A to 6C is 200V/div, respectively, whereas the unit of the horizontal axis is 100msec/div in FIG. 6A, 20 msec/div in FIG. 6B, and 2 msec/div in FIG. 6C.

Although the RF power supply oscillates at a predetermined voltageimmediately after starting oscillation, a period of several tens msec isrequired from the oscillation start to the discharge start. However, alarge voltage waveform (hereinafter referred to beard pulse”) wastransitionally observed in the moment that discharge is started as shownin FIG. 6C although it cannot be recognized in FIGS. 5, 6A and 6B.

For example, consideration is made about the plasma CVD process of a TFTdisposed on a pixel panel of the active matrix type display unit. Inorder to surely hold image data, there is required that the TFT of thepixel panel is excellent in off-state current characteristic. For thatreason, the TFT comprises, for example, the LDD structure, as shown inFIG. 1E. Because the LDD region functions as a high resistant region,the off-state current can be reduced. It should be noted that FIG. 1will be described in detail with reference to a first embodiment.

In the process of manufacturing the TFT with the LDD structure,source/drain regions 112 and 113 of an active layer 103 which is made ofsilicon are exposed as shown in FIG. 1E before forming a firstinterlayer insulating film 116 (refer to FIG. 1F). Also, because a gateelectrode 105 is not cut into respective devices, its length issubstantially identical with the width of a substrate as it is, andabout several hundreds of gate electrodes 105 with the above structureare disposed in parallel.

In the above state, in the case where the first interlayer insulatingfilm 116 is formed through plasma CVD, unless the plasma density and theplasma potential are not uniform even in the stationary plasma state,gate potential is distributed so that a current flows in the gateelectrode 105, with the result that the device is deteriorated. However,in the moment that plasma is generated, a transitional beard pulse isgenerated as shown in FIG. 6C. Further, if plasma is unevenly generated,then a current which is remarkably larger than that in the stationarystate is allowed to flow in the gate electrode.

Moreover, in an initial stage of forming the first interlayer 116,because electrons are irradiated directly onto silicon (source/drainregions 112 and 113), silicon is negatively charged. As a result,because electric field is developed in the gate insulating film 110, thegate insulating film 110 is deteriorated. However, if silicon(source/drain regions 112 and 113) is covered with the first interlayerinsulating film 116, silicon is prevented from being directly chargedup.

Therefore, in order to form the first interlayer insulating film throughplasma CVD, there arises such a problem that a transitional change involtage such as the beard pulse must be eliminated or suppressed betweenthe RF electrodes until silicon finishes being charged up.

In order to solve the above problem, according to a first aspect of thepresent invention, there is provided a plasma CVD method that increasesgradually or continuously an output of an RF power supply up to a valuewhich is at the time of forming a film.

According to a second aspect of the present invention, there is provideda plasma CVD method that continuously implements a step of generatingplasma from gas other than a raw gas, and a step of supplying said rawgas to form a film, wherein, in said plasma generating step, the outputof the RF power supply is gradually or continuously increased up to avalue which is in said film forming step.

In the conventional example, to form the silicon oxide film with TEOS asa raw material, the RF power supply is oscillated during the O₂-plasmageneration at the same output value as that during the film formation.As shown in FIG. 6A, a predetermined voltage is applied between the RFelectrodes immediately after oscillation starts. In other words, avoltage between the RF electrodes is rapidly changed.

For that reason, in the plasma CVD method with the above steps accordingto the first aspect of the present invention, the output of the RF powersupply is gradually or continuously increased up to the value which isrequired at the time of forming the film, thereby restraining a rapidand transitional change in voltage between the RF electrodes.

For example, in order to form the silicon oxide film with TEOS as a rawmaterial through plasma CVD, a step of generating plasma of O₂ and astep of supplying TEOS and generating plasma of TEOS/O₂ to form a filmare continuously conducted. Therefore, according to the second aspect ofthe present invention, in the plasma CVD method that continuouslyimplements the step of generating plasma of gas (O₂) other than the rawgas and the step of supplying the raw gas (TEOS) to form a film, in theplasma generating step when oscillation starts, the output of the RFpower supply is gradually or continuously increased up to a value whichis in the film forming step, thereby suppressing a rapid andtransitional change in voltage between the RF electrodes.

It should be noted that, in the plasma generating step, the raw gas maybe supplied simultaneously when the output of the RF power supplybecomes identical with the value which is 9 at the time of forming thefilm. Alternatively, the raw gas may be supplied while shifting a timingat which the output of the RF power supply becomes identical with thevalue which is at the time of forming the film. Further, the lower limitof the RF output when oscillation starts can be defined by adischargeable value.

On the other hand, the upper limit of the RF output when oscillationstarts may be appropriately set for each device or each reactionchamber. This is because, according to the inventors' research there isa case where no transitional change in voltage between the RF electrodesis observed, and also the probability of occurrence of the transitionalphenomenon depends on each device or each reaction chamber.

However, the voltage of the RF power when oscillation starts is half orless of the voltage of the RF power when material gas supplies.

FIGS. 3A to 3C show the waveforms of a voltage applied between the RFelectrodes during the O₂-plasma generating step when the output of theRF power supply is 50 W, which has been observed through an oscilloscopeby the present inventors. FIG. 3A shows a voltage waveform from the timepoint of starting oscillation and after starting discharge. FIGS. 3B and3C show voltage waveforms at the time of starting discharge. The momentat which discharge starts is recognizable as a shift of the waveform. Itshould be noted that the unit of the vertical axis of FIGS. 3A to 3C is200 V/div, respectively, whereas the unit of the horizontal axis is 100msec/div in FIG. 3A, 20 msec/div in FIG. 3B, and 2 msec/div in FIG. 3C.

As is apparent from comparison of FIGS. 3A to 3C with FIGS. 6A to 6C,the RF output is set to 50 W which is lower than 250 W (voltage whenforming a film) when discharge starts, thereby suppressing the rapid andtransitional change in voltage such as the beard pulse at the time ofstarting discharge (at the time of generating plasma of O₂).

FIGS. 4A and 4B show the waveforms of a voltage applied between the RFelectrodes during the O₂-plasma generating step, which has been observedthrough an oscilloscope, in which the RF output is set to 50 W whenoscillation starts, and is then increased to 250 W after discharging.FIG. 4A shows a voltage waveform from oscillation starts and thendischarge starts, and FIG. 4B shows a voltage waveform at the time ofincreasing the RF output. The moment at which discharge starts isrecognizable as a shift of the waveform. It should be noted that theunit of the vertical axis of FIGS. 4A and 4B is 200 V/div, respectively,whereas the unit of the horizontal axis is 1 msec/div in FIG. 4A, and 2msec/div in FIG. 4B. As shown in FIG. 4B, the RF output is graduallyincreased from 50 W to 250 W after discharging (after the generation ofO₂ plasma), to thereby smoothly increase a voltage between the RFelectrodes, thus being capable of suppressing the transitional change involtage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are cross-sectional views showing a process ofmanufacturing a TFT in accordance with a first embodiment;

FIG. 2 is a diagram showing the waveform of a voltage applied between RFelectrodes when forming a silicon oxide film;

FIGS. 3A to 3C are diagrams showing the waveform of a voltage appliedbetween the RF electrodes when generating O₂ plasma;

FIGS. 4A and 4B are diagrams showing the waveform of a voltage appliedbetween the RF electrodes when generating O₂ plasma in accordance with asecond embodiment; and

FIG. 5 is a diagram showing the waveform of a voltage applied betweenthe RF electrodes when forming a silicon oxide film in accordance with aconventional example; and

FIG. 6 is a diagram showing the waveform of a voltage applied betweenthe RF electrodes during a process of generating O₂-plasma.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, a description will be given in more detail of embodiments accordingto the present invention with reference to the accompanying drawings.

(First Embodiment)

A first embodiment is directed to the present invention applied to aprocess of manufacturing a TFT, and FIGS. 1A to 1F show cross-sectionalviews of a TFT in each process of manufacturing a TFT in accordance withthis embodiment. As shown in FIG. 1A, a silicon oxide film 102 with athickness of 3000 Å is deposited on a glass substrate (Corning 1737) 101as an under film through plasma CVD or sputtering Then, an amorphoussilicon film with a thickness of 200 to 1500 Å, for example, 500 Å isformed on the silicon oxide film 102 through plasma CVD method or thereduced pressure CVD.

Thereafter, the amorphous silicon film is thermally annealed so as to becrystallized. The crystallized amorphous silicon film is etched to forman active layer 103 of a TFT. As a method of crystallizing the amorphoussilicon film, there can be adopted a laser annealing method or a methodof conducting thermal annealing and laser annealing together. If using ametal element that promotes the crystallization of silicon, such as Nior Pt, the crystallizing process can be conducted at a lower temperatureand a shorter period of time.

Then, an aluminum film with a thickness of 6000 Å that constitutes agate electrode 105 is deposited through sputtering. The aluminum filmcontains scandium of 0.1 to 0.3 weight % therein. Then, an anodization(aluminum oxide) film 106 is formed on the surface of the aluminum filmthrough anodization. In this situation, a voltage of 10 to 30 V isapplied to the aluminum film in an ethylene glycol solution containingtartaric acid of 3% therein. The anodization layer 106 thus formed has afine (barrier-type) structure. Then, a resist mask 106 is formed on thesurface of the anodization layer 106, and the aluminum film is patternedto form a gate electrode 107 (FIG. 1A).

As shown in FIG. 1B, a constant voltage of 10 to 30 V is applied to thegate electrode 105 in the electrolyte solution while the resist mask 106is attached to the anodization layer, to thereby conduct anodization. Asthe electrolyte solution, there can be used an acid solution in whichcitric acid, oxalic acid or sulfuric acid is diluted to 3 to 20%. Inthis embodiment, a voltage of 10 V is applied to the gate electrode 107in oxalic acid solution (30° C.) for 20 to 40 minutes. As a result, aporous type anodic oxide 108 with a thickness of 5000 Å is formed ononly sides of the gate electrode 105. It should be noted that thethickness of the oxidation 108 may be controlled by oxidation time (FIG.1B).

As shown in FIG. 1C, the resist mask 107 is removed to secondly anodizethe gate electrode 105 in the electrolyte solution. In this situation,an ethylene glycol solution containing 3 to 10% of tartaric acid, boricacid, or sulfuric acid is used. As a result, a barrier type anodic oxide109 is formed in the periphery of the gate electrode 10. The thicknessof the barrier type anodic oxide 109 is set to 1500 to 2000 Å. Thethickness of the anodic oxide 109 may be appropriately determined by thelength of an offset and overlapping. The thickness of the barrier typeanodic oxide 109 is nearly proportional to a supply voltage, and whenthe supply voltage is 200 V, the thickness of the anodic oxide 109 is2500 Å.

Then, with the anodic oxides 108 and 109 as masks, the silicon oxidefilm 104 is etched to form a gate insulating film 110. For example, ifCF₄ is used as an etching gas, only the silicon oxide film 104 can beetched and the porous anodic oxide 108 not being etched (FIG. 1D).

As shown in FIG. 1E, impurity ions that give conductive type isimplanted into the active layer 103. In the case of forming a p-typeTFT, p (phosphorus) ions are implanted into the active layer 103, but inthe case of forming an n-type TFT, B (boron) ions are implantedthereinto. Also, in order to make the gate insulating film 110 functionas a semi-transmission mask, such conditions as the dose amount or theacceleration voltage are appropriately set.

As a result, in the active layer 103, the gate electrode 105 functionsas a complete mask, and a region just below the gate electrode 105 formsa channel formation region 111, into which no impurity ions areimplanted. Also, exposed regions of the active layer 103 form a sourceregion 112 and a drain region 113 because impurity ions with a highdensity are implanted into the exposed regions of the active layer 103.Regions which are covered with only the gate insulating film 110 formlow-density impurity regions 114 and 115 which are lower in density ofimpurity ions than the source region 112 and the drain region 113because the gate insulating film 110 functions as a semi-transmissionmask when ion implanting. In particular, a low-density impurity region115 between the channel formation region 111 and the drain region 113 iscalled “LDD region”. The impurity density or the low-density impurityregions 114 and 115 may be set to be lower than that of the source/drainregions 112 and 113 by about 2 figures (FIG. 1E).

Subsequently, a silicon oxide film 116 with a thickness of 5000 Å isdeposited as an interlayer insulator with a raw material of TEOS throughplasma CVD. In this embodiment, in order that the beard pulse as shownin FIG. 6C is eliminated or suppressed between the RF electrodes in theinitial stage of forming a film, the output of the RF power supply in aplasma CVD device is gradually increased. For that reason, the RF outputis first set to 50 W to generate O₂ plasma. Thereafter, simultaneouslywhen the RF output is increased to 250 W. TEOS gas is supplied togenerate TEOS/O₂ plasma, thereby forming a silicon oxide film 110.

FIG. 2 is a diagram showing the waveform of a voltage applied betweenthe RF electrodes when forming the silicon oxide film 116 which has beenobserved through an oscilloscope, and FIGS. 3A to 3C are waveforms of avoltage applied between the RF electrodes during the O₂-plasmageneration process. FIG. 3A is a voltage waveform from the time ofstarting oscillation and then discharge starts, and FIGS. 3B and 3C arewaveforms of a voltage at the time of starting discharge. The moment atwhich discharge starts is recognizable as a shift of the waveform. Itshould be noted that the unit of the vertical axis of FIGS. 3A to 3 c is200 V/div, respectively, whereas the unit of the horizontal axis is 100msec/div in FIG. 3A, 20 msec/div in FIG. 3B, and 2 msec/div in FIG. 3C.

Conventionally, because, in the O₂ plasma generation, the RF powersupply is oscillated at a high output as in the film formation, thebeard pulse has been observed as shown in FIG. 6C. In this embodiment,because the RF power supply is oscillated at a low output such as 50 W,the generation of the beard pulse can be suppressed at the time ofstarting discharge as shown in FIG. 3C. Therefore, because no largevoltage is transitionally applied to the RF electrodes, the device onthe substrate 101, in particular, the deterioration of the gateinsulating film 110 can be suppressed.

It should be noted that in the present invention, the RF output isgradually increased from 50 W to 250 W and then TEOS supply start.Alternatively, the RF output may be continuously increased from 50 W to250 W after discharging (after O₂ plasma is generated). In this case, incomparison with the case where the RF output is gradually increased, aabruptly change in voltage between the RF electrodes can be moresuppressed. Also, when the RF power supply of the plasma CVD device issubjected to lamp-up (slow start), the transitional fluctuation ofvoltage can be further suppressed between the RF electrodes.

As shown in FIG. 1F, contact holes are formed after forming the siliconoxide film 116, a titanium film and an aluminum film are continuouslyformed and patterned to form source/drain electrode/wiring 117 and 118.Through the above processes, a TFT having the LDD structure isfabricated.

In this embodiment, the process of forming the silicon oxide film 116which constitutes an interlayer insulating film has been described.Similarly, in other plasma CVD process, for example, in the process offorming the silicon oxide film 102 that constitutes the gate insulatingfilm 110, a film may be formed while the RF output is controlled asdescribed above.

(Second Embodiment)

In the first embodiment, when forming the silicon oxide film 116, the RFoutput is reduced during the O₂ plasma generation process, and the RFoutput is increased to a predetermined value simultaneously when TEOS issupplied. In this embodiment, the RF output is increased to the samevalue as that of forming a film. In other words, in this embodiment, theRF power supply is oscillated at an output of 50 W to generate O₂plasma. After a predetermined period of time has elapsed, the RF outputis increased up to 250 W, and then TEOS is supplied to start the filmformation.

FIGS. 4A and 4B are diagrams showing the waveform of a voltage appliedbetween the RF electrodes when forming the silicon oxide film accordingto this embodiment which has been observed through an oscilloscope,which is a voltage waveform during O₂ plasma generation. FIG. 4A shows avoltage waveform from oscillation starts and then discharge starts, andFIG. 4B shows a voltage waveform at the time where the RF output isincreased from 50 W to 250 W. The moment at which discharge starts isrecognizable as a shift of the waveform. In this embodiment, becauseO₂-plasma is generated when the RF output is 50 W as in the firstembodiment, no beard pulse is observed before and after discharge startsalthough being not recognizable in FIG. 4A. Also, although the RF outputis increased from 50 W to 250 W after discharging, a voltage between theRF electrodes is smoothly increased to cause no transitional change involtage as shown in FIG. 4B.

Therefore, with the application of the process of forming the filmthrough plasma CVD in accordance with this embodiment, an electricstress can be prevented from applying to the device on a substrate.

It should be noted that in the present invention, the RF output isgradually increased from 50 W to 250 W after discharging (after thegeneration of O₂-plasma) during the O₂-plasma generation process.Alternatively, the RF output may be continuously increased from 50 W to250 W after discharging (after O₂-plasma is generated). In this case, incomparison with the case where the RF output is gradually increased, achange in voltage between the RF electrodes can be more suppressed.Also, when the RF power supply of the plasma CVD device is subjected tolamp-up (slow start), the transitional fluctuation of voltage can befurther suppressed between the RF electrodes.

Also, in this specification, the process of manufacturing the TFT on theglass substrate has been described. However, it can be applied to aprocess of fabricating a semiconductor device/circuit prepared on asilicon wafer.

As was described above, according to the plasma CVD method of thepresent invention, because the RF power supply starts to be oscillatedat an output lower than that when forming a film, a voltage between theRF electrodes when starting discharge can be prevented from changingtransitionally and drastically. Hence, because the devices that failduring the plasma CVD process are reduced, the yield can be improved.

Further, the present invention can be readily realized by only changinga method of controlling the RF power supply in the plasma CVD device,for example, by changing the RF voltage.

1. (Canceled)
 2. A plasma CVD method characterized in that an output ofan RF power supply is gradually or continuously increased to a valuewhich is at the time of forming a film.
 3. The method of claim 2,wherein an initial voltage of the RF power is half or less of a voltageat forming the film.
 4. The method of claim 2, wherein said filmcomprises an interlayer insulating film of a thin film transistor.
 5. Aplasma CVD method comprising the steps of: generating plasma from afirst gas by an RF power; and supplying a second gas to form a filmduring generating the first gas plasma, wherein said RF power isgradually increased and said first gas does not form a deposit by itselfupon decomposition thereof.
 6. The method of claim 5, wherein a firstvoltage of the RF power at generating plasma is half or less of a secondvoltage at supplying the second gas.
 7. The method of claim 5, whereinsaid first gas comprises oxidizing gas and said second gas comprisesorganic silane.
 8. The method of claim 5, wherein said film isinterlayer insulating film of a thin film transistor.
 9. A method formanufacturing a semiconductor device comprising: forming a semiconductorfilm over an insulating surface; forming a gate insulating film over thesemiconductor film; and forming a gate electrode over the gateinsulating film, wherein the gate insulating film is formed by plasmaCVD in which an output of an RF power supply is gradually orcontinuously increased to a value for forming the gate insulating film.10. A method for manufacturing a semiconductor device comprising:forming a TFT over an insulating surface; and forming an interlayerinsulating film over the TFT; wherein the interlayer insulating film isformed by plasma CVD in which an output of an RF power supply isgradually or continuously increased to a value for forming theinterlayer insulating film.
 11. A method for manufacturing asemiconductor device comprising: forming a semiconductor film comprisingpolycrystalline silicon: forming a gate insulating film over thesemiconductor film comprising polycrystalline silicon; forming a gateelectrode over the gate insulating film; and forming an interlayerinsulating film over the gate electrode and the gate insulating film,wherein the interlayer insulating film is formed by plasma CVD in whichan output of an RF power supply is gradually or continuously increasedto a value for forming the interlayer insulating film.